Prior art vinylidene fluoride base fluororubbers are used in a variety of applications mainly in the automobile and mechanical industries since they are elastomers having good heat resistance, chemical resistance and mechanical strength.
These fluororubbers, however, are not satisfactory in chemical resistance. They are readily swollen in polar solvents such as ketone, lower alcohol, carbonyl and organic acid solvents. They are deteriorated by amine-containing chemicals so that their rubber strength and elongation may substantially decline. With respect to low-temperature properties, the fluororubbers lose rubber elasticity at temperatures below −20° C. to such an extent that they cannot be used as sealants. This generally imposes a limit on the use of fluororubbers in cold areas.
In order to eliminate these drawbacks, curable fluorine-containing compositions comprising a perfluoro compound and a fluorinated organohydrogenpolysiloxane as main components were proposed. These compositions are liquid because they are based on liquid perfluoro compounds having a low degree of polymerization. Then the compositions are suitable in the formed in-place gasket (FIPG) process and liquid injection molding system (LIMS), but less effective in compression molding conventionally employed for rubber molding.
In particular, the curable fluorine-containing compositions often prohibit the use of conventional two-plate molds for rubber because of the difficulty of molding and the frequent occurrence of defects by air entrapment. Steady production is difficult unless a special LIMS mold is newly fabricated. The LIMS mold, however, has the problems that it is generally more expensive than the conventional two-plate molds for rubber, the mounting of the mold in a LIMS molding machine is cumbersome, and the molding machine requires time-consuming adjustment after mold mounting. The LIMS mold does not lend itself to the manufacture of a variety of parts in small quantities.
Under the circumstances, we previously proposed in U.S. Pat. No. 6,297,339 or EPA 0967251 a method for preparing a rubber composition of the type that can be roll milled and molded in a compression molding rubber mold, referred to as millable type composition, from a fluorinated curable composition comprising a perfluoro compound, a fluorinated organohydropolysiloxane, a filler and a surface treating agent.
This rubber composition is satisfactory when roll milled in a small scale, but in a mass production scale using 5 kg of more, generates heat during roll milling operation so that its temperature elevates. The heated composition becomes more sticky to the roll surface and interferes with roll working. When the rubber composition is held for a long period of time after the roll working, it changes its viscosity and becomes less flowable.
Continuing research works, we discovered that the roll working efficiency on a mass production scale of such rubber compositions can be improved using special surface treating agents. See JP-A 2001-106893.
It would be desirable if roll working efficiency was improved without a need for special surface treating agents.